PURPOSE: To evaluate the dioptric power of the normal central cornea. METHODS: Retrospective analysis of combined slit-scanning and Placido-ring topography in 30 normal eyes. RESULTS: Keratometric-mean, anterior-mean, thickness-mean, and total-mean powers; keratometric-optical and total-optical, keratometric-axial and total-axial, and total-tangential powers; equivalent power and the addition of anterior and posterior mean powers, were similar (p>0.8361) in the four analyzed central areas with 0.04, 1.0, 3.0, and 5.0 mm diameter. The posterior-mean power decreased (p=0.0030) from -6.62 D to -6.35 D. The average thickness increased (p=0.0231) from 524 µm to 554 µm, representing 0.13 D of total-mean corneal power. Keratometric-optical (p<0.0167), keratometric-axial (p<0.0099), and keratometric-mean powers (p<0.0030) were approximately 1.5 D more positive than the respective total powers. Total-mean power was equal to equivalent power (p>0.4907), and to the addition of anterior-mean and posterior-mean powers (p>0.3868). Anterior-mean power was about 7.5 greater than posterior-mean power. CONCLUSIONS: The real power of the cornea calculated using both surfaces, its thickness, and the physiologic refraction indices is smaller (less positive) than the power usually accepted and determined by the anterior corneal surface radius and the keratometric index of refraction (F<K). Anterior-mean/posterior-mean power ratio is not ten-to-one as conventionally assumed. Power of posterior surface and thickness vary according to the size of the analyzed area. However, power of anterior surface and total power of the cornea remain the same. Components of total corneal power and pachymetry seem to be independent of each other.
Cornea; Corneal topography; Ocular refraction; Intraocular lenses
